Physiological and genetic studies indicate that senescence is a highly regulated process (Nooden, Senescence and Aging in Plants, (L. D. Nooden and A. C. Leopold, Ed.), pp. 391-439, Academic Press, San Diego, Calif., 1988; Thomas, et al., Ann. Rev. Plant Physiol. 31:83-111, 1980). Molecular studies suggest that changes in gene expression are associated with the senescence program. For example, the level of mRNA encoding proteins involved in photosynthesis decrease during senescence (Bate, et al., J. Exp. Bot. 42:801-811, 1991; Hensel, et al., Plant Cell 5:553-564, 1993; Jiang, et al., Plant Physiol. 101:105-112, 1993), while mRNA levels of genes encoding proteins thought to be involved in the senescence program increase (Graham, et al., Plant Cell 4:349-357, 1992, Hensel, et al., Plant Cell 5:553-564, 1993; Kamachi, et al., Plant Physiol. 93:1323-1329, 1992; Taylor, et al., Proc. Natl. Acad. Sci. USA 90:5118-5122, 1993).
It has been suggested that senescence specific promoters can be used to drive the expression of select genes during senescence. U.S. Pat. No. 5,689,042, for example, utilizes a genetic construct comprising a senescence specific promoter, SAG12, operably linked to a Agrobacterium isopentyl transferase (IPT)-coding DNA sequence not natively connected to the promoter sequence. Transgenic plants comprising this construct retain green leaves longer by driving the expression of IPT by means of the SAG12 promoter. IPT is known to increase the level of cytokinin, a class of plant hormones the concentration of which declines during senescence and thus may play a role in controlling leaf senescence.
Similarly, Gan and Amasino show that inhibition of leaf senescence can be achieved by autoregulated production of cytokinin (Gao, et al., Science 270:1986-1988, 1995). Other senescence-inducible promoters have been identified. For example, the SARK promoter from Phaseolus vulgaris is described in WO 99/29159 and Hajouj et al. Plant Physiol. 124:1305-1314 (2000).
A useful and desirable aspect of internally regulating the expression of the gene of interest is in the ability to regulate the expression only in those cells undergoing senescence thus leaving normal cells unaffected and spared from the possibly negative effects of cytokinin overproduction.
Although the use of SAG12 controlled expression of IPT has been shown to control leaf senescence, other phenotypes of such plants are not well understood. The present invention addresses these and other needs.